The black-headed caterpillar Opisina arenosella (Lepidoptera: Xylorictidae) is a new serious pest of coconut tree in Ben Tre province in recent years. Contribute to finding an eco-friendly measure to manage this pest, several experiments on the effectiveness of Bacillus thuringiensis var. aizawai (Bt.a) strain were carried out at the Plant Protection Department, College of Agriculture under Can Tho University. The results show that the B. thuringiensis var. aizawai at the concentration of 107 CFU/ml and 108 CFU/ml reached 66.67% and 78.33% in 7 days after treatment, respectively. The LC50 value of Bt.a train on second instar were 4.5x106 ; 2.8x104 and 1.2x104 CFU/ml at 4, 6 and 7days after treatment, respectively, the lethality time (LT50) also decreased with increasing concentration of bacteria, the concentration of 107 CFU/ml and 108 CFU/ml gave equally LT50 values, ranging from 5.1 to 5.267 days. Regarding to liquid and powdered Bt.a preparation, dose of 1.5 l/ha and 2.0 kg/ha showed the efficiency reached 63.65% and 74.76%, respectively at 7 days after treatment.